We have demonstrated that highly palatable, "junk-food" diets reliably induce obesity in rats. When these diets are removed, feeding and obesity are reduced in animals on high-carbohydrate chow but not to the same extent in animals on high-fat chow. Removal of insulin (streptozotocin diabetes) does not alter the reduction in feeding on high-carbohydrate maintained obese animals. Raised insulin levels do, however, limit feeding in normal-weight animals. Rats receiving two units or regular insulin per day via steady infusion, lost weight and reduced feeding by decreasing meal size but not frequency. Rabbits and rats receiving single exogenous glucagon injections have also been shown to limit feeding over the short term (first hour postinjection) by limiting meal size and lengthening intermeal interval. Glucagon was effective only in glycogen-repleted (free-fed) and vagal-intact animals. Vagotomy also reduces daily feeding in rabbits by limiting ingestion rate within meals and meal size. Infant rats suppress feeding to glucose and galactose early in life (preweaning) while enhanced feeding to decreased glucose utilization (2 DG, insulin, phloridizin) does not appear until the fifth week. Increased glycemia in response to 2 DG seems unimpaired despite hypothalamic isolation by knife transection.